SAT2数学考试题目：Fractions

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　　Being able to efficiently and correctly manipulate fractions is essential to doing well on the Math IC test. A fraction describes a part of a whole. It is composed of two expressions, a numerator and a denominator. The numerator of a fraction is the quantity above the fraction bar, and the denominator is the quantity below the fraction bar. For example, in the fraction 1 /2, 1 is the numerator and 2 is the denominator.

　　Equivalent Fractions

　　Two fractions are equivalent if they describe equal parts of the same whole. To determine if two fractions are equivalent, multiply the denominator and numerator of one fraction so that the denominators of the two fractions are equal. For example, 1/2 = 3/6 because if you multiply the numerator and denominator of 1 /2 by 3, you get:

　　As long as you multiply or divide both the numerator and denominator of a fraction by the same nonzero number, you will not change the overall value of the fraction. Fractions represent a part of a whole, so if you increase both the part and whole by the same multiple, you will not change their fundamental relationship.

　　Reducing Fractions

　　Reducing fractions makes life with fractions a lot simpler. It takes unwieldy fractions such as 450 /600 and makes them into smaller, easier-to-work-with fractions.

　　To reduce a fraction to its lowest terms, divide the numerator and denominator by their GCF. For example, for 450 /600, the GCF of 450 and 600 is 150. So the fraction reduces down to 3?4.

　　A fraction is in reduced form if its numerator and denominator are relatively prime (their GCF is 1). Thus, it makes sense that the equivalent fractions we studied in the previous section all reduce to the same fraction. For example, the equivalent fractions 4/6 and 8/12 both reduce to 2/3.

　　Comparing Fractions

　　When dealing with integers, large positive numbers with a lot of digits, like 5,000,000, are greater than numbers with fewer digits, such as 5. But fractions do not work the same way. For example, 200/20,000 might seem like a big, impressive fraction, but 2 /3 is actually larger, because 2 is a much bigger part of 3 than 200 is of 20,000.

　　In certain cases, comparing two fractions can be very simple. If the denominators of two fractions are the same, then the fraction with the larger numerator is bigger. If the numerators of the two fractions are the same, the fraction with the smaller denominator is bigger.

　　However, you’ll most likely be dealing with two fractions that have different numerators and denominators, such as 200/20,000 and 2/3. When faced with this situation, an easy way to compare these two fractions is to utilize cross-multiplication. All you have to do is multiply the numerator of each fraction by the denominator of the other, then write the product of each multiplication next to the numerator you used to get it. We’ll cross-multiply 200/20,000 and 2/3:

　　Since 40,000 > 600, 2 /3 is the greater fraction.

　　Adding and Subtracting Fractions

　　On SAT II Math IC, you will need to know how to add and subtract two different types of fractions. Sometimes you will be given two fractions with the same denominator, and other times you will have two fractions with different denominators.

　　Fractions with the Same Denominators

　　Fractions can be extremely easy to add and subtract if they have the same denominator. In addition problems, all you have to do is add up the numerators:

　　Subtraction works similarly. If the denominators of the fractions are equal, then you simply subtract one numerator from the other:

　　Fractions with Different Denominators

　　If the fractions do not have equal denominators, the process becomes somewhat more involved. The first step is to make the denominators the same, and then to subtract as described above. The best way to do this is to find the least common denominator (LCD), which is simply the LCM of the two denominators. For example, the LCD of 1/2 and 2/3 is 6, since 6 is the LCM of 2 and 3.

　　The second step, after you’ve equalized the denominators of the two fractions, is to multiply each numerator by the same value as their respective denominator. Let’s take a look at how to do this for our example, 1/ 2 + 2 /3. For 1/2:

　　So, the new fraction is 3 /6. The same process is repeated for the second fraction, 2 /3:

　　The new fraction is 4 /6. The final step is to perform the addition or subtraction. In this case, 3/6 + 4/6 = 7/6.

　　If you think it will be faster, you can always skip finding the LCD and multiply the denominators together to get a common denominator. In some cases, such as our example, the product of the denominators will actually be the LCD (2 3 = 6 = LCD). But, other times, the product of the denominators will be greater than the LCD. For example, if the two denominators are 6 and 8, you could use 6 8 = 48 as a denominator instead of 24 (the LCD).

　　The drawback to this second approach is that you will have to work with larger numbers and reduce your answer in the end.

　　Multiplying Fractions

　　Multiplying fractions is quite simple. The product of two fractions is the product of their numerators over the product of their denominators. Symbolically, this can be represented as:

　　Or, for a numerical example:

　　Dividing Fractions

　　Multiplication and division are inverse operations. It makes sense, then, that to perform division with fractions, all you have to do is flip the second fraction , which is also called taking its reciprocal, and then multiply.

　　Here’s a numerical example:

　　Mixed Numbers

　　A mixed number is an integer followed by a fraction, like 11/ 2. It is another form of an improper fraction, which is a fraction greater than one. But operations such as addition, subtraction, multiplication, or division can only be performed on the improper fraction form, so you need to know how to convert between mixed numbers and improper fractions.

　　Let’s convert the mixed number 11 /2 into an improper fraction. First, you multiply the integer portion of the mixed number by the denominator, and add that product to the numerator. So 1 2 + 1 = 3, making 3 the numerator of the improper fraction. Now, simply put 3 over the original denominator, 2, and you have your converted fraction.

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